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All the World's a Stage ... for Dust

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All the World's a Stage
... for Dust

Tune in to a NASA website and watch giant dust clouds as they ride global rivers of air, cross-pollinating continents with topsoil and microbes.

Marshall Space Flight Center

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June 26, 2001 -- People may think of the ground under their feet as being stationary, but the soil and dust of the world is constantly on the move, blown aloft by the wind -- sometimes across entire oceans! 

Now, anyone with access to the World Wide Web can watch airborne dust migrate around the globe simply by pointing their browser to the aerosol homepage for NASA's Total Ozone Mapping Spectrometer, or "TOMS" for short. Although the primary function of TOMS is to monitor the ozone layer, it also measures aerosols -- that is, airborne dust, smoke, and other particulates. NASA scientists use these data to create daily maps and movies that they post online.

Above: This TOMS aerosol movie, which spans the interval June 13 through 21, 2001, shows a cloud of African dust crossing the Atlantic and raining bits of the Sahara Desert over the Caribbean. Click on the image to view a larger 450 kb animation, which includes a color table and legend. See also the latest TOMS maps and more information about the color-coded aerosol index.

Our planet's atmosphere provides a transcontinental highway for dust that's been stirred up from dry soils by strong winds. Because dust particles are so small -- often less than 0.002 mm across -- they can remain aloft for days as they ride global rivers of air. Larger sand grains don't get airborne as often or for as long, but they can be pushed along the ground by the wind or washed away by water erosion. 

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This constant reshuffling of the world's sand and dust ties the continents together and serves as a reminder that, in the natural world, there are no political boundaries. Airborne microbes and pollen in Florida or Brazil might have come from Africa. Mineral dust in the soils of India could have blown over from Iran.

In fact, over vast stretches of geologic time, the action of wind thoroughly mixes and re-mixes the world's dust. So the soil in your own backyard might contain some grains of dust from places all over the globe!

While much of this reshuffling of earth can't be seen by satellites, dust often migrates in huge clouds that show up in satellite images.

For example, the Americas frequently inherit huge volumes of dust from Africa and Asia, while countries of the Far East like India and China get sprinkled with dust from the Middle East.

African dust plumes begin their trans-Atlantic journey with storm activity in the Sahara Desert region. The dust, originating from fine particles in the arid topsoil, is transported into the atmosphere by winds and may be carried more than 10,000 feet high. Dust clouds cross the Atlantic Ocean and reach the Caribbean and the Americas in about 5 to 7 days.

see captionHow much dust is transferred by these clouds? One estimate places the number at about 13 million tons of dust each year drifting from Africa to the Northeastern Amazon Basin alone!

Right: This TOMS image shows a record-setting Asian dust cloud beginning its journey east across the Pacific. Click on the image to view a 440 kb animation of the dust cloud migrating to North America. Remember that as the dust disappears from the satellite's view, it's raining out of the air onto the ocean and the land. See a similar Asian dust cloud from 1998 in this animation.

"In some cases, such as in the Caribbean islands, much of the dust in the top soil is from Africa," says Jay Herman, principle scientist for aerosol detection by TOMS at NASA's Goddard Space Flight Center. "Over generations, lots of it piles up there. The top soil has been analyzed on these islands, and it's definitely of African origin."

Northern spring also brings massive dust clouds blowing east over the Pacific Ocean from China and Mongolia. The largest Asian dust cloud on record blew all the way across North America earlier this year, traveling as far east as the mid-Atlantic Ocean before finally disappearing from view.

"The huge amount of dust blowing off Asia recently is kind of a new phenomenon," Herman says. "It's apparently due to increased desertification brought about by a change in the dryness of the area and increased land use."

Usually, dust clouds from Asia don't reach so far, and much of the dust simply falls into the Pacific Ocean. Asian dust clouds don't make it to the Americas as often as African clouds do for two main reasons, Herman says. First, the season for dust-carrying winds is much shorter in China than it is in Africa. Second, the sources of Asian dust -- such as the Gobi and Takla Makan deserts -- are much smaller than the African sources, which include the Sahara Desert and the ever-shrinking Lake Chad.

Below: The hazy streets of Baichen in the Jilin Province of northern China appear somewhat apocalyptic on April 7 during the peak of the violent dust storms that gave birth to the record-setting Asian dust cloud. Click on the image for a larger version. Photo by Zev Levin.

see captionThe raining of dust into the Pacific Ocean may be important for the productivity of fisheries in the North Pacific, an important source of seafood for much of the world. Photosynthetic microbes in the top layer of the ocean form the foundation of the food chain. Often the concentration of dissolved mineral nutrients (particularly iron) controls the abundance of these microbes. Some of the minerals in the dust might help boost microbe populations and, therefore, fish populations. Scientists are still working to verify this effect, however.

Airborne dust clouds can also affect local weather by suppressing rainfall. Cloud droplets form around the minute dust grains, but some kinds of dust limit how big these droplets can become. Because droplets must reach a certain size before they'll fall to Earth as rain, dust clouds can leave dry weather in their path.

In addition to dust, the TOMS instrument detects volcanic ash, pollutants, smoke, ozone concentrations and ultraviolet light intensities. Most of these data are available online in near real-time.

Remarkably, the sensor is able to make all of these measurements by simply observing the sunlight reflected toward it from the Earth. "It looks at the Earth just as you would with your eye to see how bright it is in the ultraviolet wavelengths," explains Herman. "If you could see in the ultraviolet, you'd basically be seeing a color change when aerosols are present. That's all the measurement is." 

see captionRight: Dry soil on the move. Dust storms can be a frightening sight -- they kick up walls of dust often hundreds of feet high that sand-blast everything in their path. Some of this dust could get lofted up to higher altitudes, where streams of air could carry it hundreds of miles and deposit it on distant soils.

To detect aerosols, the TOMS instrument records two particular wavelengths of ultraviolet (UV) light: 331 nanometers and 360 nanometers. Some of the UV light reflecting from Earth is absorbed by the dust and other aerosols. Clouds reflect both wavelengths almost equally, while aerosols cause absorption of the shorter wavelength more than the longer one. This difference allows scientists to distinguish aerosols from clouds.

"We put data on our website every day, usually within minutes of receiving it," Herman says.

A new TOMS instrument is scheduled to launch on August 10 aboard the QuikTOMS satellite. This satellite will replace the aging TOMS instrument currently in orbit, and while both instruments are functional, the redundant data will help NASA scientists to calibrate the new one. QuikTOMS's expected mission life is 3 years.

Thanks to TOMS and its successor, we'll continue to have a front row seat on the global stage of migrating dust for years to come.

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Web Links

Today's aerosol conditions -- daily updates of aerosols (airborne dust and other particles) around the globe, from the Total Ozone Mapping Spectrometer (TOMS) on NASA's Earth Probe satellite

Total Ozone Mapping Spectrometer (TOMS) -- homepage from Goddard Space Flight Center

QuikTOMS -- homepage from GSFC

April 2001 Asia dust event -- information about the largest dust cloud to cross the Pacific Ocean from Asia on record, from the TOMS website

Mobile Homes for Microbes -- Science@NASA story African dust that crosses the Atlantic and brings beautiful sunsets to Florida also carries potentially harmful bacteria and fungi, a new study shows.

The Pacific Dust Express -- Science@NASA story North America has been sprinkled with a dash of Asia! A dust cloud from China crosed the Pacific Ocean recently and rained Asian dust from Alaska to Florida.

Dust Begets Dust -- Science@NASA story Everyone knows that dry weather leads to dusty soils, but new research suggests that dust might in turn lead to dry weather.

Watching Wildfires from Space -- Science@NASA story Find out more about Earth Probe TOMS and how it keeps track of dust and smoke in Earth's atmosphere.

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